Natural Laminar Flow (NLF) on an aerodynamic surface requires a high quality surface finish if premature transition is to be avoided. The presence of waviness, roughness, gaps and steps in the surface can be a challenge in meeting the surface quality criteria.
In the case of an aircraft wing, waviness is one property that is significantly dependent upon flight loads and the wing skin must be sized to meet stringent deflection criteria at cruise conditions.
Typically, the internal leading edge region of an aircraft wing is vented to remove fumes (e.g. from the aircraft internal wing fuel tanks) and water build up.
The skin panel of a wing leading edge traditionally is held in place by leading edge ribs and the skin thickness is sized to account for bird strike requirements. For the particular application of Natural Laminar Flow (NLF) strict control of the surface waviness is required to avoid premature transition.
If the leading edge region is vented on the wing lower (high pressure) surface, as is normal, the internal pressure within the leading edge region at cruise conditions is such as to cause a significant pressure load across the skin panel which in part defines the wing upper (low pressure) surface of the leading edge. This leads to excessive waviness and premature transition. A typical solution is to increase the thickness of the wing skin but this incurs a significant weight penalty.